Tyndall Glacier is a small cirque glacier in Rocky Mountain National Park, Colorado, on the east side of the Continental Divide just north of Hallett Peak. It is unusual because it combines features of both an ice glacier and a rock glacier, with lower reaches composed largely of rock debris and only a small amount of ice. The glacier is named for Irish scientist and mountaineer John Tyndall, who made the first ascent of the Weisshorn and attempted the Matterhorn in 1861. Its name has added significance because Tyndall identified carbon dioxide as a heat-trapping greenhouse gas that same year, and the glacier appears to be retreating.